An anti-checkpoint role for Aspergillus nidulans snoARif1

Eukaryotes respond to DNA damage by activating two major damage-sensing pathways, mediated by ATM and ATR. Once DNA damage is recognized, ATM and ATR phosphorylate, and thereby activate, CHK2 and CHK1 kinases, respectively. Together, CHK1 and CHK2 kinases activate cell cycle checkpoints by phosphorylating a variety of effectors in order to halt DNA synthesis, prevent mitosis, and induce DNA repair. Acting in opposition to these two checkpoint enforcers, the telomere homeostasis regulator, RIF1, in budding yeast may act as an ‘anti-checkpoint’ protein to fine-tune the threshold for ssDNA recognition and to facilitate recovery from checkpoint arrest. In budding yeast, Rif1 anti-checkpoint activity was limited to damage at telomeric DNA sites. Furthermore, an anti-checkpoint role for Rif1 has not been demonstrated in higher eukaryotes. In the filamentous fungus Aspergillus nidulans, we are using chkA/Chk1, chkB/Chk2, and the DNA synthesis regulator nimO/Dbf4 to unravel the anti-checkpoint function of the Rif1 ortholog, snoA (suppressor of nimO). In this study, we show that defects in snoA partly alleviate the DNA damage sensitivity of cells lacking chkA (ΔchkA), and largely reverse the checkpoint defect in cells deleted in the BRDF checkpoint domain (BRCT and Dbf4-similarity domain) of nimO. Whereas nimOΔBRDF ΔchkA double mutants confer synthetic lethality, loss of snoA restores viability to these strains. This suggests that snoA may act as a global anti-checkpoint regulator in A. nidulans. To test this idea further, we are continuing to study genetic interactions in cells that lack both major DNA damage response pathways mediated by chkA and chkB.